Since its introduction in the early 80's the compact disc (CD) has firmly established as a popular format for digital optical storage. The digital versatile disc, also known as the digital video disc (DVD) is now emerging as a versatile medium and is believed to eventually replace the CD. The capacity of a DVD is seven times that of the CD. The increased capacity is primarily achieved through the use of shorter wavelength laser and narrower track-pitch. However, because of their very strong wavelength dependence, dyes for recordable CD (CD-R) application at 780 nm are not generally extendible to recordable DVD (DVD-R) that operates around 635 nm wavelength.
It has been disclosed in commonly-assigned U.S. Patent No. 5,585,158 and U.S. patent application Ser. Nos. 08/631,352 filed Apr. 12, 1996, entitled "A Method of Forming Recordable Optical Element Using Low Absorption Materials" by Raychaudhuri et al; 08/631,754 filed Apr. 12, 1996, entitled "A Method of Forming Recordable Optical Element Using Low Absorption Material" by Raychaudhuri et al; 08/732,825 filed Oct. 15, 1996, entitled "Recording Media for Recordable Element" by Raychaudhuri et al; and 08/902,544, filed Jul. 29, 1997, entitled "High Performance Media for Optical Recording" by Olin et al, that optical recording thin layers based on the Ge--Te--C--H--O system can be appropriate as recording layers in CD-R optical recording elements. These layers exhibit a weak dependence of optical properties to wavelength, and with only minor alteration of composition and thickness believed usable as DVD-R optical recording elements as well.
Optical recording elements were fabricated of optical recording thin layers comprising one or more sublayers each containing the said components with a reflector layer of gold and a protective overcoat as shown in FIG. 1. The composition and thickness of each of the sublayers being selected to approximately correspond to the R.sub.min reflectivity and further having the optical recording elements properties modified by heat treatment. The optical recording elements thus prepared have been reported applicable to CD-R and also believed useful in DVD-R applications.
Since CD-R and DVD-R are aimed at the consumer market it is highly desirable to reduce cost. While attempting to replace the reflecting layer of gold with less expensive metals, it was found that silver provided the required reflectivity that appeared stable at room temperature. At elevated temperature aging, near 95.degree. C., the reflectivity of disc (measured through the substrate) decreased while the reflectivity of Ag (measured through transparent protective overcoat) stayed unchanged. This indicates that the interaction at the recording layer/silver interface is the main cause of instability.